A device for measuring easily the inside of living body without causing damages to the living body is hoped for in such fields as clinical medicine, brain science and the like. For example, when specifically the head is considered as the subject of measurement, brain diseases such as cerebral infarction, brain hemorrhage, etc. and high-order brain functions such as thinking, language, physical exercise and the like are mentioned. And such subjects of measurement are not limited to the head, but also preventive diagnosis relating to heart diseases such as cardiac infarction and the chest region, visceral diseases of the kidney, liver and the like in the abdomen are mentioned. When the head is considered as the subject of measurement and diseases in the brain or high-order brain functions are measured, it is necessary to clearly specify the involved area or the functional region. For this reason, it is very important to measure a wide area of the head as an image.
For the requirements mentioned above, optical measurements are very effective, because the normal and abnormal state of living body organs and the activation of the brain relating to high-order brain functions are closely related with the oxygen metabolism and blood volume within the living body. These oxygen metabolism and blood volume correspond to the density of specific pigments (hemoglobin, cytochrome aa3, myoglobin, etc.) in living body, and this pigment density is calculated by the amount of light absorption in the visual—infrared wavelength range. Devices for measuring the inside of living body by irradiating light in the visually infrared wavelength range on living body and detecting the light reflected by the living body are described for example in JP-A-Sho57 (1982)-115232, JP-A-Sho63 (1988)-260532, JP-A-Sho63 (1988)-275323 and JP-A-Hei5 (1993)-317295.
So far, time-division multiplex modulation system, frequency multiplexing system and the like among light-intensity modulation systems have been used for the measurement of light in living body. The basic structure is that of deriving intensity data by means of optical detection with the help of a lock-in amplifier, and time-division multiplexing or frequency multiplexing has been adopted to increase points of measurement.
On the other hand, an art based on CDMA (code division multiplex access) is described in JP-A-2004-333344. This is constituted by the use of Hadamard codes, the implementation of a CDMA modulation (intensity modulation), and the separation of light source and emission of light on the light receiving side. This art enabled to use the CDMA method for calculating the inside of living body.